The present invention relates to a short overall length imaging lens system with four lenses, especially to an imaging lens system for mobile phones or mini-cameras with image sensors such as CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor). The imaging lens system is formed by four lenses and is with features of short overall length and low cost.
The electronics available now become more compact and multifunctional. Most of them such as digital still cameras, PC (personal computer) cameras, network cameras and mobile phones, even persona digital assistants (PDA) are equipped with an image lens system. The imaging lens system not only requires good imaging quality but also needs compact volume (short overall length) and lower cost. Moreover, the imaging lens system with larger field angle can improve image quality of the electronics and match user's requirements.
The imaging lens system applied to mini-size electronics includes various designs such as two lenses, three lenses, four lenses or five lenses. In consideration of imaging quality, multiple lenses system has better performance in aberration correction and MTF (modulation transfer function) so it is applied to electronics with requirement of high pixel count.
In recent years, the requirements for the imaging lens system applied to high level mini-cameras, camera phones or PDA (personal digital assistant) etc. include compact volume (short overall length), good aberration correction, high pixel count and low cost. In compact design of the imaging lens system with four lenses, prior arts disclosed such as US2007/0081259, U.S. Pat. No. 7,177,098, US2003/0161051, EP1868021, EP1387199, JP2005-164899, JP2007-322844, JP2007-065374, JP2003-270530, TW I254140, TWM313780, TWM 313245, CN1573407, CN1873461, CN1892279 etc., good aberration correction is achieved by the combinations of lenses with various positive power or negative power. Moreover, designs with positive refractive power of first lens, negative of second lens, positive of third lens and negative of fourth lens were disclosed as, US2007/014033, US2008/0024882, U.S. Pat. No. 7,215,492, U.S. Pat. No. 7,321,474, EP1821129, JP2007-225833, JP2008-020893, JP 2007-286153, JP 2007-193195, TWM314860, CN1815287, etc., to achieve good aberration correction. Especially on the fourth lens, the area near the optical axis on the object sides is a convex surface while the surface turns into a concave surface at the edge of the lens and change of the curvature is quite large. Or the area near the optical axis on the image sides is a concave surface while the surface turns into a convex surface at the edge of the lens. Such lens is quite difficult to be produced. Once the glass material is used, shape of the optical surface is difficult to control while being ground or molded. When the lens is produced by plastic injection molding, there are still some problems such as cooling shrinking or others. In order to overcome difficulties in manufacturing, the fourth lens with simple optical surface improves the yield rate, as shown in US2007/0058256, US2007/0070234, US 2007/0242370, US2008/0043346, JP2005-091666, JP2005-025174, JP 2004-233654, and P2007-219520 etc.
The users are in urgent need of the imaging lens system with short overall length, good aberration correction, high pixel count and low cost. However, the overall length of the imaging lens system of prior arts requires further minimization. In order to achieve lost cost, the lens is made from material with lower refractive index instead of the material with high refractive index and high cost. Once an image side of the fourth lens is a concave surface, flare or ghost image is reduced. The yield rate is also improved if the lens includes a simpler optical surface.